In an electronic blood pressure meter for detecting blood pressure values from a finger, the finger is, first of all, placed in a finger cuff for applying pressure to the finger, and blood pressure values are then computed from the change in the amplitude of pulse wave obtained from the finger as the cuff pressure is gradually reduced, according to a certain algorithm applied to a series of pulse wave data. In such an electronic blood pressure meter, pulse wave is typically detected with a photoelectric sensor including a light emitting element for projecting light onto the finger and a light receiving element for receiving the light reflected from the finger. When blood flow in the finger is in an interrupted state, the reflected light received by the light receiving element consists of a DC signal or, in other words, has a constant level. On the other hand, as the cuff pressure is gradually reduced and blood starts flowing through finger arteries, the volume of the arteries changes and the reflected light accordingly undergoes a pulsating change. Thus, the signal obtained by the light receiving element comprises a DC component and a pulsating AC component superimposed thereon. In an electronic blood pressure meter, a pulse wave component is extracted from the output of the light receiving element and it is used as an important parameter in determining blood pressure values.
Since various properties of a finger, such as color, diameter, the distance between skin surface and arteries, texture, etc. differ a great deal from one person to another and the reflected light obtained by the light receiving element, in particular its DC component accordingly varies to a large extent. Therefore, the individual variation of the DC component may exceed the linear range of the pulse wave detecting means. In other words, the output of the light receiving element may saturate if the amplitude of the reflected light is too large, but the S/N ratio may be excessively small if the amplitude of the reflected light is too small. In either case, a satisfactory measurement of blood pressure is not possible